The present invention is concerned with glasses for ophthalmic applications. More particularly, this invention is concerned with glasses which permit the fabrication of multifocal lenses by one of the processes currently utilized; that is to say, by soldering or "fusion" of one or more glasses to the principal glass, said "segment glasses" having a higher refractive index. This invention is related to the principal glass.
This invention is designed to produce glasses having a refractive index (n.sub.d) between about 1.599-1.602, an Abbe number (.nu..sub.d) of 40.5-41.5, a softening point or Littleton temperature of 670.degree.-680.degree. C., very low birefringence when sealed to existing segment glasses for "white" glasses with a refractive index of 1.523, a density of 2.64-2.66 g/cm.sup.3, very good transmission in the visible region of the radiation spectrum (transmission at a wavelength of 400 nm for a thickness of 10 mm greater than 83%), an absence of undesirable yellow coloration, excellent chemical durability and stability with respect to devitrification, and a viscosity at the highest temperature of devitrification (or "liquidus") equal to or greater than 3000 poises so as to enable products to be produced using a continuous melting unit and formed using an automatic pressing process, which is indispensable for obtaining a cost of production that is low and compatible with the application.